Fix OpenBSD build warning

Fix OpenBSD build warning

Fix this warning:
  CC    savevm.o
/src/qemu/savevm.c: In function `do_savevm':
/src/qemu/savevm.c:1900: warning: passing arg 1 of `localtime_r' from incompatible pointer type

It looks like on OpenBSD the type of tv_sec in struct timeval is still
'long' instead of time_t as in most other OS. Fix by adding a cast.

Signed-off-by: Blue Swirl <blauwirbel@gmail.com>